Linear actuator or generator with rods

ABSTRACT

The invention concerns an electrical machine, whereof the active part includes a global solenoid winding ( 110, 210, 310 ) for each phase and includes, inside said winding(s), a stack of ferromagnetic or non-magnetic parts ( 400 ) and magnetized parts ( 500 ), the ferromagnetic or non-magnetic parts being provided with passages traversed each by at least one relatively sliding element ( 600 ), each sliding element constituting a succession of portions alternately magnetic ( 620 ) and non-magnetic ( 630 ), the passages formed in the ferromagnetic or non-magnetic parts of the stack forming orifices whereof the internal cross-section encloses each time one sliding element, the sliding elements ( 600 ) consisting in rods having each an outer periphery matching the internal cross-section of the traversed orifices.

The invention relates to electrical actuators or generators,particularly when they are designed to supply or accept a very highforce density, in other words a large force in a small volume.

In general, a mechanical drive often requires that a large thrust forceshould be obtained without heavy or large systems.

This is the case for transport equipment (aircraft, trains).

For example, the platform with variable inclination (of a swing-typetrain is actuated by jacks that must satisfy these requirements.Similarly, active compensation for vibrations in any type of transportequipment requires the use of small but powerful jacks, with highdynamic performances.

Hydraulic or pneumatic jacks, or rotating electrical actuators requiringa rotation/translation type movement transformation (typically using ascrew-nut assembly) are used in normal actuator applications with highforce density.

A force density of 300 N of generated force/liter of volume occupied istypically obtained with electrical direct linear actuators (without anyrotation-translation movement conversion), and this force density isoften too low.

Furthermore, the various indirect electrical actuators or jacksavailable in the past have serious disadvantages including limitationsof the mechanical pass band and controllability, severe mechanical wearand noise constraints, the need for a pressurized fluid supply with thecomplexity inherent to such a supply, and more generally a low globalenergy efficiency.

In the field of electrical actuators, it has been proposed to increasethe surface areas of air gaps, namely magnetic interfaces between fixedand mobile parts.

However, in this type of actuator, typically called multi-air gaps withglobal winding, it is found that the mechanical strength of mobile partsis particularly low, due to their small cross-section and guidancedifficulties that are particularly difficult to overcome.

The purpose of this invention is to solve the disadvantages of knowntechniques, in other words to propose an electrical actuator withseveral mobile elements, with a particularly high. force density andwith a high mechanical reliability while facilitating mechanicalguidance.

This purpose is achieved according to the invention by using anelectrical machine forming an actuator or generator comprising an activepart with one or several phases designed to be connected to anelectrical source or load, and a passive part, these two parts beingfree to move with respect to each other, the active part including aglobal solenoid winding for each phase and comprising a stack offerromagnetic or non-magnetic parts and magnetized parts inside this orthese windings, the magnetized parts having magnetization directionsparallel to the relative displacement direction and successivemagnetization directions opposite to each other, the ferromagnetic ornon-magnetic parts of this stack being provided with passages eachcarrying at least one element free to slide with respect to the activepart, this or these sliding elements forming the passive part, eachsliding element including a succession of alternately magnetic andnon-magnetic portions arranged to be facing the different magnetized ornon-magnetized part of the stack one after the other, such that analternating magnetic flux is generated in the winding in each phase,characterized in that the passages formed in the ferromagnetic ornon-magnetic parts of the stack form orifices for which the internalsection surrounds a sliding element each time, and in that the slidingelements are rods each of which has an external periphery complementaryto the internal section of the orifices through which they pass, suchthat each rod interacts magnetically with the ferromagnetic ornon-magnetic part through which its external periphery passes.

Other characteristics, purposes and advantages of the invention willbecome clear after reading the following detailed description given withreference to the attached figures on which:

FIGS. 1 and 2 are partial longitudinal sections through the deviceillustrating two successive positions of the device, referred to aspositive conjunction and negative conjunction respectively.

FIG. 2 b is is a detailed description of an elementary pattern of apowered part of the electrical machine in FIGS. 1 and 2;

FIG. 3 is a perspective general sectional view of an electrical machineaccording to the invention;

FIG. 4 is an exploded view of a portion of a mobile element of anelectrical machine according to the invention;

FIG. 5 is an exploded view of a portion of the active part of theelectrical machine according to the invention;

FIGS. 6 and 7 diagrammatically represent an electrical machine accordingto the invention, in two positions in which the forces produced are inopposite direction to each other;

FIG. 8 is a sectional perspective view of a position sensor according toa preferred embodiment of the invention;

FIG. 9 is an exploded view of this sensor;

FIG. 10 is an overview of an electrical machine according to theinvention illustrating the position of a support for the sensor in FIGS.8 and 9;

FIG. 11 is a view of a sensor support according to an embodiment of theinvention;

FIG. 12 is a plot illustrating the variation of the magnetic inductanceof the primary winding of such a sensor as a function of the position ofa corresponding mobile element;

FIG. 13 is an electrical diagram of a differential measurement deviceprocessing and analyzing output signals from three such sensors.

This device is naturally reversible provided that the load and thesource are also reversible.

Motor type operation occurs when there is an electromechanicalconversion between a source and a load, the source being electrical innature (in which case we refer to an electrical power supply) and theload being mechanical in nature (in which case we refer to the drivenload). Generation type operation occurs when the source is mechanical innature (driving mechanical source) and the load is electrical in natureand absorbs electrical power.

Secondly, the device comprises two parts, firstly a part comprisingelectrical windings and permanent magnets and a completely passive partcomposed of rods, each part being fixed or mobile. Therefore we willtalk about a relative sliding movement to qualify all possibilities ofrelative movement between the two parts.

For easier understanding, we will consider the case in which theelectrical machine is operating as a motor (or actuator) and the mobilepart corresponds to the rods. The comments and characteristics that willbe described will thus be equally valid for operation as a generatorand/or with a fixed passive part.

The electrical machine described herein is composed of an active partpowered by one or several power windings (windings 110, 210 and 310 inFIG. 3), called armature windings. These windings are in solenoid formand completely surround one zone of the powered part and the passivepart.

Apart from these windings, the powered part comprises a stack ofmagnetic or non-magnetic plates and permanent magnets 500.

The passive part is composed of one or several elongated elements 600,the shape of which will be described later.

Unlike conventional structures (particularly with rotating fields), theknown “multi-air gaps with global winding” technology can be used toreduce the displacement pitch without correspondingly degrading thetotal converted energy. Therefore, this can increase the generatedforce, for a given total volume and for given thermal and magneticconditions.

The objective in this case is to maximize the sum of the air gapsurfaces, in other words the active external surface areas of the rods,with respect to the volume of the actuator considered.

To achieve this, this example embodiment includes a series of rods eachof which has a circular periphery that interacts with each plate orferromagnetic or non-magnetic part around its entire periphery.

In other words, the magnetic fluxes transported by the sheet metal partsare applied to the rod around its entire periphery.

The main advantage of this shape lies in the manufacturing precision andthus the possibility of facilitating mechanical guidance while enablingsmall air gaps.

An ovoid section rod, or more generally a rod totally surrounded byferromagnetic or non-magnetic zones themselves in contact with themagnets, would also advantageously generate maximum variation ofmagnetic flux with respect to the extent of the cross-section throughthis rod.

In order to make the magnets 500 and the magnetic or non-magnetic parts400 that entirely surround the cylindrical rod 600 considered, the partpowered in this case is partly a composite assembly composed of a stackof magnetic or non-magnetic plates and permanent magnets, and thepermanent magnets surround each rod with a shape complementary to theperiphery of the rod.

This stack is parallel to the displacement of the rod and alternatesalong this direction.

The distance defined by the sum of the thicknesses of a magnetic ornon-magnetic plate 400 and the next magnet 500, corresponds to ahalf-relative displacement pitch of a rod.

More precisely, as shown in FIG. 3, the powered part is composed ofthree identical phases (q=3) offset in space (in the displacementdirection) by a number equal to N*pitch/(2*q), where N is an integer.

Each phase is composed of:

a power winding 110, 210, 310 (called the armature) in the shape of asolenoid winding surrounding the active zone of the powered part;

a stack of magnetic or non-magnetic circuits (plates) 400 and permanentmagnets 500 with magnetization parallel to the displacement. This partcomprises circular holes through which the mobile parts 600 pass;

magnetic flux return circuits: two end circuits 120, 130 and an tubularshaped outer circuit 140, all surrounding the windings and the activezone.

The passive part is composed mainly of several parallel rods 600 movingrelative to each other through the powered part.

The rods 600 in this case are also composite and comprise a non-magneticcore on which a stack of magnetic and non-magnetic washers are arrangedsuccessively as will be described later. But other manufacturingprocesses could also be envisaged provided that an appropriatelongitudinal magnetic-non-magnetic alternation is obtained.

The operating principle of the electrical machine is similar to theprinciple of synchronous machines with permanent magnets.

In a manner known in itself, and particularly using the known principleof synchronous electrical machines, displacement of the mobile parts 600generates an alternating flux and consequently an alternatingelectromotive force (emf) within the power winding.

Based on the same logic, injection of an alternating current synchronouswith this emf into the winding (the armature) generates anelectromagnetic power with a non-zero average value and therefore anaction driving force on the mobile elements and a reaction force on thefixed part.

There are therefore two characteristic positions of the mobile part,shown in FIGS. 1 and 2; a positive conjunction position (FIG. 1) inwhich the flux seen by the armature winding and generated by the “activemagnets” 500 is arbitrarily positive, and a negative conjunctionposition (FIG. 2) in which this flux is negative.

The magnets 500 are initially positioned such that their orientationdirection is alternating.

Consequently, one magnet 500 out of two is inactive.

The flux from inactive magnets does not pass through the correspondingpowered winding but is short circuited by the magnetic part of the rodthat is facing it.

The rod 600 is composed of alternately magnetic and non-magneticelements with a rod pitch equal to twice the pitch of the magnets.

Each magnet 500 shown in FIG. 5 is a ring with an internal cross-sectionconforming with the periphery of a rod that passes through it, but thisis only one of several possible embodiments.

Each basic element of the stack making up the powered part thereforecomprises:

a magnetic or non-magnetic sheet metal wall 400 with a series oforifices distributed around the edge of its periphery;

and a set of ring magnets 500, each covering the edge of thecorresponding orifice.

Therefore all ring magnets thus distributed also form the magnetizedspacer that separates two magnetic or non-magnetic sheet metal plates400.

According to one variant, instead of inserting ring shaped magnetsbetween two magnetic or non-magnetic plates, a single magnet is insertedwith the same shape as the sheet metal plates, in other words a magnetin the form of a plate through which several crossing orifices aredrilled for each corresponding rod 600.

As shown in FIG. 4, the rods 600 themselves have a particularlyadvantageous composite structure. Each rod consists of a successivestring of rings on a central non-magnetic core.

Thus a non-magnetic rod 610 receives a magnetic ring 620 and anon-magnetic ring 630 with the same diameter alternately, eventuallyforming a compact rod with an appropriate magnetic structure perfectlycomplementary to the orifices in the plate 400. Therefore, according tothis advantageous arrangement, each of these rods consists of a stack offerromagnetic parts 610 and non-magnetic parts 620.

In another variant, they may also form a single piece with alternatelymagnetic and non-magnetic zones.

It should be noted that apart from providing a very high force density,this embodiment also provides excellent mechanical dynamics, in otherwords a good capacity for acceleration and a wide pass-band, and finallya long machine stroke.

The low inertia of the mobile parts is particularly conducive towardsthis mechanical dynamics, the low inertia being particularly due to thefact that in this case the mobile part is completely passive, since thefixed part comprises both the power winding and the field coil (in thiscase the permanent magnets).

The operating mode according to the principle of a synchronous actuatorwith permanent magnets, variable local reluctance with longitudinalfield and global winding is preferred with the invention.

In particular, this particular arrangement results in completely passiveand consequently low volume and lightweight rods.

The air gap surface areas necessary to obtain very high force densitiesare increased, due the cylindrical surface area of the rods.

It should be noted also that the cylindrical structure of the rods alsoenables precision machining, and precise assembly and guidance.

The cylindrical structure also enables particularly advantageousguidance by sliding strips.

It should be noted also that the fact that the armature completelysurrounds the passive part (in other words that the winding can bequalified as “global”) also makes the device compact.

This device also comprises a force transmission system made such thatall rods 600 are only stressed in tension, regardless of the directionof the generated force that contributes to the production of a deviceseparated into a very large number of small section rods;

FIGS. 6 and 7 show a preferred variant of such a force transmissionsystem.

Firstly, the Figures show a spindle 700 for transmission of theresultant force generated by the actuator. This spindle 700 extends notonly outside the machine, but also passes through the complete machine.

In this embodiment, the spindle 700 is fixed to two bearing plates 710and 720 transverse to this rod, positioned on each side of the fixedactive part.

The rods 600 transmit the force onto these plates 710, 720 in order toactuate the main spindle 700.

To achieve this, each rod 600 passes through the two plates 710 and 720to form a head 640, 650 beyond each plate and opposite the active part,preventing it from being extracted from the corresponding plate.

When the force transmitted on the spindle 700 is a thrust, in otherwords it is in the direction of the actuated load, the rod heads 640adjacent to the load tend to separate from the plate 710 (this tendencyis symbolized by a separation in the Figures), while the heads 650furthest from the load apply a force in contact with the correspondingplate 720, and push it towards the load.

On the other hand, when the force on the spindle 700 is a tension force,the rod heads 650 furthest from the load tend to separate from theircorresponding plate 720 (symbolic separation in the Figures) and therods are tensioned on the load side through the heads 640 in contact, onthe plate 710 nearest to the load.

Once again, the entire force is transmitted to the spindle 700 only byapplying tension to the rods 600. The force is transmitted on the endheads of the only plate located on the tensioned side of the rod.

In this example embodiment, there is no play under the rod heads fromthe plate considered, such that relief of the thrust force at one end ofthe rods does not necessarily cause separation of the rod headsconsidered.

However according to one variant, the lengths of the rods 600 may bechosen to be slightly greater than the gap between the two plates 710and 720, such that there is a slight longitudinal clearance between thehead 640, 650 of each rod 600 considered, and each corresponding forcetransmission plate 710, 720.

Therefore the rods are stressed in tension only regardless of thedirection of the force, which relieves mechanical stresses in the rod(and prevents buckling). The extent of the cross-sections of mobileelements (rods and others) can then be reduced compared with what isused in a conventional transmission system. Thus, the number of rodsthat can be arranged in a particular volume may be high. The forcedensity is increased correspondingly.

In other words, forces in tension rods only make it possible to use rodswith a particularly small cross-section, enabling a large number of rodsand the corresponding advantages.

Furthermore, this machine is provided with a position sensor 800 that isintegrated into the structure and does not oversize the machine, suchthat the overall volume of the assembly is not increased. This sensor,which is very much appreciated for auto-control of the machine, is ofthe variable reluctance type. This technology has the advantage ofhomogeneity with the machine technology, thus contributing to thepossibility of operation in difficult environments.

It is composed of a primary excitation winding 810 and a secondarymeasurement winding 820. As shown in the FIGS. 8 and 9, this sensor usesthe general shape of the rods 600 described above. The two windings 810and 820 each form a ring solenoid both inside a cover 830.

The cover itself is in the form of a hollow ring, and the ring assembly800 thus formed has an inside diameter complementary to the diameter ofthe rod 600 when the rod is assembled.

The sensor 800 is inserted around the rod in an end area of theactuator, into which the rod slides during operation of the actuator.

The reluctance (and therefore the inductance) of the sensor 800 dependson the position of the mobile rods 600. Injection of a current into theprimary 810 of the sensor, with an appropriate frequency and amplitude,generates an electromotive force at the secondary 820, the modulus ofwhich depends particularly on the value of the inductance andconsequently the position of the rod 600.

Thus, the rod position information can easily be extracted by applyingan appropriate electronic and/or digital processing to the measurementof the measured electromotive force (FIG. 13 shows an example of adifferential measurement device).

In its three-phase version, the actuator comprises three positionsensors of this type arranged at the end of the actuator on threedifferent rods, and each dedicated to controlling one of the threephases. Each sensor 800 is offset in the direction of displacement ofthe rods, by a number equal to N*pitch/(2*q), where N is an integer andq is the number of phases.

In FIG. 12, the inductance seen by the windings 810 and 820 of thesensor as a function of the position of the rod 600 that it surrounds,is approximately in the shape of a sine curve, such that taking accountof this sinusoidal inductance in three offset positions with the sameoffset as the different phases, provides a means of deducing the preciseposition of the actuator at each instant.

A sensor holder 900 like that shown in FIG. 11 is used to position thethree sensors in this way, and is composed of a plate made of anon-magnetic material with three through passages in which each of thethree passages also forms a sensor housing cavity 910, a cavity 910 witha variable depth, the cavity depth corresponding to the position chosenfor the sensor. The difference in cavity depths is equal to one third ofthe pitch between the rods.

In the diagram in FIG. 13, the device for differential processing ofsignals output by the sensors 800 is based on the principle of the samealternating current power supply (excitation frequency) for each of thethree sensors (the primary windings of the three sensors are thus putinto series), power supply obtained by current regulation of a powersupply consisting of a corrector 900, starting from the signal output bya sine generator 1000 and by an alternating power supply 1100. The sinegenerator signal is used to demodulate (synchronous demodulator 1200)the output signal recorded on a sensor 800 and subjected to acorresponding low pass filter 1300 so as to provide the position signalat the output from each low pass filter 1300, the variation of thesignal being perfectly synchronous with the electromotive force of thephase considered (machine power winding). Taking account of each ofthese three sinusoidal signals provides a means of measuring therelative position of all rods (passive part) with respect to each of thethree phases (three-phase version) in real time.

The sensor as described herein avoids conventional position sensors, forexample of the opto-magnetic type, that usually limit the usageenvironment, generate pollution, and increase the cost of the assembly.

The fact that the position sensor uses mobile parts of the actuator isalso an advantage in itself.

In its three-phase version, the actuator is powered by a conventionalthree-phase power inverter with six power switches and its three phasesare preferably coupled in star formation. Seen from the power inverter,its operation is similar to the operation of a conventional synchronousmachine with permanent magnets, with the sole difference that magneticcouplings between phases are very low or even negligible.

The many advantages provided by the various arrangements described willhave been made clear from the presentation of the particular embodimentdescribed above.

The tests carried out on this actuator have demonstrated that a totalforce of 1270 N can be supplied for an rms current of 15 A.

Therefore, the corresponding force density is 1.2 kN/liter, and it wouldeven be possible to achieve a density of 2 kN/liter in pulse mode usingthe configuration described herein. Significant progress can still bemade because the structure has not yet been optimized.

Apart from the applications mentioned in the preamble to thisdescription, it is also worth mentioning other possible uses, such as amechanical drive application for manipulating industrial robots, forexample for use in the automobile industry, in addition to aircraftcontrol surface applications.

Other example applications for which this invention is very usefulinclude applications requiring a large mechanical pass-band such asactive compensation of vibrations in traction (urban vehicles, highspeed trains).

1. An electrical machine forming an actuator or generators comprising an active part that includes one or several phases designed to be connected to an electrical source or load, and a passive part, the active part and passive part being free to move with respect to each other, the active part including a global solenoid winding for each phase and including a stack of ferromagnetic or non-magnetic parts and magnetized parts inside the windings, the magnetized parts having magnetization directions parallel to a relative displacement direction and successive magnetization directions opposite to each other, the ferromagnetic or non-magnetic parts of the stack including passages that each carry at least one element free to slide with respect to the active part, the sliding elements forming the passive part, each sliding element including a succession of alternating magnetic portions and non-magnetic portions arranged to be facing different magnetized or non-magnetized parts of the stack one after the other, such that an alternating magnetic flux is generated in the winding in each phase, characterized in that the passages formed in the ferromagnetic or non-magnetic parts of the stack form orifices with an internal section that surrounds a sliding element wherein the sliding elements are rods each of which has an external periphery complementary to the internal section of the orifices through which they pass, wherein each rod interacts magnetically with the ferromagnetic or non-magnetic part through which its external periphery passes.
 2. An electrical machine according to claim 1, wherein the orifices have a circular cross-section and each rod is in the form of a rotating cylinder.
 3. An electrical machine according to claim 2, wherein each of the rods includes a longitudinal groove.
 4. An electrical machine according to claim 1, wherein the machine is adapted so that the number of phases is equal to q and each phase is offset from a mobile part in the displacement direction by a distance such that each of the magnetic fluxes is offset by a number equal to Nπ/q, where N is an integer.
 5. An electrical machine according to claim 1, wherein each magnet is formed by a ring surrounding a corresponding rod.
 6. An electrical machine according to claim 1, wherein each magnet is formed by a magnetized plate perpendicular to the displacement, provided with a series of orifices, so that a corresponding rod may pass through each orifice.
 7. An electrical machine according to claim 1, wherein each rod includes a central non-magnetic core and a series of alternately magnetic and non-magnetic rings.
 8. An electrical machine according to claim 1, wherein the machine includes one or more annular positioning sensors that include one or several windings, wherein the sensors are fixed to the active part so that one or several rods of the passive part may pass through them.
 9. An electrical machine according to claim 8, wherein each of the positioning sensors is located around a rod and offset in the direction of displacement of the active and passive parts with respect to each other.
 10. An electrical machine according to claim 1, wherein the machine includes at least one device for collection of forces applied by at least one rod, wherein the rod and the device are adapted to cooperate by applying a single-directional contact along a tension direction of the rod, wherein the force collection device does not create any compression reaction on the rod.
 11. An electrical machine according to claim 10, wherein there is a clearance between the contacts of the rods and the force collection devices.
 12. An electrical machine according to claim 10, wherein the force collection device is a transverse plate fixed to a main spindle of the machine, wherein each rod passes through the plate and is provided with a bearing head in contact with the plate.
 13. An electrical machine according claim 1, wherein the rods have a cross-section, wherein the circumference of a rod forms a smooth line with no abrupt change of direction. 